LaSalle County is a county located within the Fox Valley and Illinois River Valley regions of the U.S. state of Illinois. As of the 2020 Census, it had a population of 109,658. Its county seat and largest city is Ottawa. LaSalle County is part of the Ottawa, IL Micropolitan Statistical Area of Northern Illinois. LaSalle County borders Woodford, Marshall, Putnam, Bureau, Livingston, Lee, DeKalb, Kendall, and Grundy counties.